1. Field of the Invention
The present invention relates generally to signal detecting apparatus and more particularly to a method and apparatus for detecting and displaying a particular type of periodically recurrent signal in a data stream. A specific example of a signal to be detected is the synchronizing "S" signal used in an ISDN communication network.
2. Brief Description of the Prior Art
In order to make a stream of analog or digital data intelligible, it is necessary that some type of reference component be inserted into the data stream so that it can be detected and then used as a benchmark from which to interpret the information content of the stream. A classical example of such a data stream in the analog domain is a television signal which includes sync pulses used to identify the start of each scan line. In the digital world, synchronizing frame bits are used to identify the beginning and ending of data words containing a plurality of digital bits of data.
In communications networks, the accuracy and efficiency of the medium is directly related to the ability of the network to faithfully communicate the synchronizing signals. In an attempt to standardize communications links, media and interfaces, the CCITT, a communications standard group that is part of the United Nations, has defined a recommendation for a worldwide Integrated Services Digital Network (ISDN) capable of handling voice and data over copper wires, fiberoptics, satellite channels and other implementations of future technologies.
Because such a system must encompass high-level data such as video images, computer messages, voice communications, and other information, and must also specify low-level data concerning wires, connectors, frequencies, voltages, etc., the system has been designed in "layers" with the bottom layer (layer 1) representing the physical phenomenon, and the top layer (layer 7) representing user applications. In between are layers that partition the network in terms of well-defined interfaces that range from the interface at the bottom (physical) layer, over which physical signals are passed, up an increasingly abstract hierarchy to the most general, "application", layer which represents the user's desired task or application that makes use of the digital (ISDN) communications network. The CCITT layers are rigorously defined at the interfaces between layers, and the messages that flow between the layers are also specified.
It is important to note that the implementation of the layers is not specified, leaving complete freedom to the designer of the layered communication system. In use, messages flow down from the top layers to the physical layer, across the network, and up to the "peer" layer at the destination.
While the seven-layer scheme is designed to allow any computer to communicate with any other computer, regardless of make or manufacture, the top layers are not absolutely essential to successful communications across the network. The bottom three layers, i.e., the Physical (1), Data Link (2), and Network (3) layers, are essential, and must exist where any device or system is capable of communicating across the ISDN.
The ISDN network recommended by the CCITT committee uses a four-wire connection from the network to the subscriber defined as the "S" interface. The signals flowing between the Subscriber (S) and the Network Terminator (T) points are depicted in FIGS. 1 and 2 of the drawing.
According to the CCITT recommendation, and as particularly depicted in FIG. 1, a pseudo-ternary code with 100% pulse width is used on the "S" interface. An example of pseudo-ternary code is shown at 10. A logical 1 corresponds to a neutral level (no current), whereas logical 0's are coded as alternating positive and negative pulses.
The frame structures of the signals flowing from network to subscriber, and from subscriber to network are shown in FIG. 2. One S-frame consists of 48 bits at a nominal bit rate of 192 kbps. Thus, each frame carries two octets of data corresponding to a first channel B1, two octets of data corresponding to a second channel B2, and four channel identifying D-bits, according to the B1+B2+D structure defined for the ISDN basic access (total useful data rate: 144 kbps). The start of a frame is marked using a code violation in the form of a framing bit "F" which, because of its shape, is referred to herein as the "S" signal, or frame sync signal.
Although an acceptable pulse shape is defined by a "template", including framing limits which bracket the tolerance limits of the S-shaped pulses, most pulses are measured and displayed in terms of "eye" diagrams. This is partly due to the fact that since there is no CCITT-defined method of implementing the waveform, there is no standard way to synchronize to the "S" signal for measurement purposes. However, from the Frame Structure diagram shown in FIG. 2, it can be determined that one of the 48 bits, i.e., the bit 12, crossing the "S" interface every 250 microseconds (i.e. 8,000-48 bit frames/sec) has a defined transition, all other bit transitions depending on B-channel, D-channel, balance bits and other bits preceding the transition.
Because the frame balancing begins anew with each frame, the balance bit (L) that immediately follows the "high" frame sync (F) bit must be "low" to balance the DC charge build-up on the line. The voltage waveform at the framing bit as received at the subscriber-receiver terminal will therefore have the forms illustrated in FIG. 3. Note that on each side of the framing bit "F" there is a DC balancing bit "L", the polarity of which will change dependent upon the data that appears in the D and B channels. If no data appears within the frames, the states of the balance bits will alternate, as depicted at 14 and 16, in order to maintain DC balance on the line. The alternating "fat top", "thin top", "fat top" . . . characteristic will therefore make it difficult to use a simple oscilloscope-type technique to lock onto and display the condition of the framing bit.